The lake was actually done twice. Always keen to try new things, I decided first to use Woodland Scenics E-Z Water, which comes in the form of pellets that are melted in a small pan and poured, or sprinkled in place and melted with a hair dryer. I chose the latter technique, with dismal results. First, the hair dryer melted some of the surrounding tree foliage. And the pellets formed a perfectly placid puddle that wasn't at all realistic.
I attempted to improve it by applying a layer of textured thick gloss medium; the results were only marginally better. But then things went downhill quickly: the lake developed a large crack down the middle, and the layer of gloss medium began to peel off.
After tearing out the gloss medium and recreating a new shoreline with dirt and talus, I sealed the E-Z Water with acrylic paint, then applied a deep purple, almost black finish that faded into colors matching the shoreline. Then I finished it off with two coats of thick gloss medium textured into waves. The results exceeded my expectations.